I think the point being made is that America made harsher pollution laws to fix pollution internally, with the side effect that pollution was exported to factories that did the manufacturing (and polluting) in other nations.

Until very recently (in historic terms), America was the consumer of a large share of goods produced in those markets.

So I don't think you can say technology improvements are a prime reason for mfg pollution dropping in America. It is A reason, but not the only.